The most common key layout for a computer keyboard is known as the QWERTY keyboard, so named because the first six letters are q, w, e, r, t and y. The keyboard was originally designed for use with mechanical typewriters that used downwards force on keys to actuate pivoted typeset to impact against an inked ribbon. The QWERTY keyboard was specifically designed to reduce the speed of a typist because the typewriters suffered from jamming when typists struck keys in quick succession on more intuitive keyboard layouts, such as those with an alphabetic layout.
Clearly such a mechanical limitation is not a problem with electric typewriters, word-processors or computer keyboards, yet the QWERTY key layout has persisted in these devices. The persistence is even more remarkable for compact devices, such as smart phones and tablet devices, on which users type with a single finger, one or two thumbs or one-handed. This is true of both physical keyboards and virtual keypads (generated on a touchscreen using software).
The problem applies also to other character sets, such as keypads using other Roman-based alphabets (Spanish, French, Indonesian, etc.), non-Roman alphabets (Greek, Cyrillic, Korean, That, etc.), or character based scripts such as Japanese and Chinese.